Large construction machines having hydraulically controlled implements often include one or more variable displacement hydraulic pumps being driven by an internal combustion engine. As the operator manipulates the hydraulically controlled implement through one or more levers or other input devices, the hydraulic system responds by directing hydraulic fluid flow to the appropriate hydraulic circuits. Thus, the operator requests the implement to move in a desired direction at a prescribed velocity and to apply a desired amount of force by manipulating the appropriate input device.
As requested hydraulic effort increases, the hydraulic control system increases the displacement of the variable displacement hydraulic pump such that the amount of hydraulic flow increases. Since the amount of torque required to drive the hydraulic pump is a function of pressure and flow, as flow and pressure increase, a higher load is applied to the internal combustion engine. Thus, engine load is a function of hydraulic flow and pressure.
Under many operating conditions, the amount of hydraulic power exceeds the amount of power the engine is capable of producing at that engine speed. When this occurs, the rotational speed of the engine decreases along its lug curve. This condition is known in the industry as engine lug. In the extreme, the engine may actually stall if the requested power becomes too high.
To avoid stalling the engine, skilled operators typically reduce the amount of power being requested by the hydraulic system when they sense a loss of engine speed. While this action avoids engine stall, even skilled operators overcompensate and therefore unnecessarily reduce the amount of hydraulic work the implement is truly capable of performing. As a result, machine productivity is reduced.
Fuel mixture combustion during engine lug also becomes less efficient resulting in increased transient emissions and reduced fuel economy. It is therefore desirable to eliminate engine lug to reduce emissions and fuel consumption.
Some level of engine lug, however, is desired by operators because it provides an indication that the machine is operating at maximum capacity. Without engine lug, it is very difficult for an operator to recognize that the maximum amount of work is being performed. The degree of lug must therefore be managed to provide an indication that maximum work effort is being expended while limiting emissions and fuel consumption.
Prior art systems have not fully addressed the problem at hand since the prior art controls do not include input from all relevant parameters. The typical hydraulic system is complex and includes a number of parameters that may be sensed to provide indications of operating conditions. As is well-known in the art of control systems, the overall system can be controlled more efficiently and effectively if several key parameters are used by the control algorithm.
Prior art hydraulic system controls have not fully integrated engine speed governing with many of the available sensed parameters. For example, some prior art systems control the amount of fuel injected into each cylinder in response to only engine speed or in response to only discharge pressure without responding to both of the above parameters simultaneously. No prior art utilizes turbocharger boost pressure and/or pump swasher displacement. Similarly pump displacement has been altered in response to engine speed deviation and pump discharge pressure while ignoring boost pressure. Even though these systems are moderately effective, such systems fail to maximize system efficiency and make precise control of engine speed difficult due to problems of engine speed and/or pump displacement overshoot and oscillation.
Referring to FIG. 1, an ideal engine underspeed control system either eliminates engine lug altogether whereby engine speed will never droop substantially below S1 or allows engine speed to reduce directly from S1 to S0 with a minimum of overshoot and oscillation, where S0 is maximum engine horsepower. In relatively primitive systems using only a single sensed parameter such as engine speed, overshoot and oscillation are typically very large and are represented by the plot trace dropping initially from S1 to S3. The more measurements of key system parameters are used and the more responsive the control is to these parameters, the more overshoot and oscillation can be reduced. An intermediate system is represented by the plot trace that drops initially from S1 to S2 and ultimately settles at S0. As described above, S0 can be selected to be at a level that provides an indication of maximum power output without reducing fuel consumption and increasing emissions significantly.
The present invention is directed towards overcoming one or more of the problems set forth above.